MONDAY, July 15 (HealthDay News) -- If battling a deadly disease can be said to have a silver lining, this might be it: Many forms of cancer appear to lower the risk for developing Alzheimer's disease, new research suggests.

After sifting through the health records of nearly 3.5 million patients, investigators concluded that most kinds of cancer seem to confer some degree of protection against Alzheimer's, reducing risk of the age-related brain disorder by anywhere from 9 percent to 51 percent.

And they have also linked a common form of cancer treatment -- chemotherapy -- to a lower risk for developing Alzheimer's-related dementia.

"We found that the majority of cancers were associated with a decreased risk of [Alzheimer's disease]," said study lead author Dr. Laura Frain, a geriatrician with the VA Boston Healthcare System. "This does not mean that if you have cancer you won't get [Alzheimer's], but that you may have a decreased risk, depending on the cancer type."

Chemotherapy conferred additional protection against Alzheimer's in most cancers, with the exception of prostate cancers, Frain said. "Our findings suggest that some chemotherapies may have a neuroprotective action. Further studies are needed to confirm this," she added.

Frain and colleagues are scheduled to present their findings Tuesday at the Alzheimer's Association international conference in Boston. The research uncovered an association between some cancers and possible protection from Alzheimer's, but it did not prove the existence of a cause-and-effect relationship.

Frain's findings come on the heels of another large study published online July 10 in Neurology, in which an Italian team also unearthed a potential protective link between cancer and Alzheimer's disease.

In that case, investigators identified an inverse relationship between cancer and Alzheimer's disease, in which having cancer appeared to lower the risk for Alzheimer's by 35 percent, while having the progressive brain disease lowered the risk for cancer by 43 percent.

For the current effort, Frain's team pored through the medical paperwork of millions of American veterans who moved through the VA health care systems between 1996 and 2011.

All the patients were over 65 and dementia-free when they first sought medical attention.

On average the veterans were tracked for just shy of six years, during which time more than 82,000 were diagnosed with Alzheimer's. About one-quarter of these patients also had one of 19 different types of cancer. However, roughly three-quarters did not.

Although not all cancers were associated with lower Alzheimer's risk, many were. Having liver cancer was linked to a 51 percent drop in Alzheimer's risk, while pancreatic cancer was linked to a 44 percent drop. Esophageal cancer, myeloma, lung cancer and leukemia were also associated with a lower risk for Alzheimer's, in the range of 33 percent to 23 percent. What's more, such risk reduction could not be explained by the premature death of cancer patients, the study authors said.

However, melanoma, prostate cancer and colorectal cancers were not found to have any protective relationship regarding Alzheimer's risk. Nor was cancer generally linked to a reduced risk for developing other common age-related health complications.

Indeed, cancer patients appeared more likely to experience stroke, osteoarthritis or eye problems, such as cataracts. The majority of cancer patients also appeared to face a higher risk for forms of dementia other than Alzheimer's.

In terms of cancer treatments, undergoing radiation was not linked to reduced Alzheimer's risk. But undergoing chemotherapy lined up with a drop in Alzheimer's risk ranging from 20 percent to 45 percent.

Frain said the research team is now investigating which chemotherapy drugs are associated with a lower risk of Alzheimer's disease.

"The benefit of investigating this unusual, inverse relationship between cancer and [Alzheimer's] may be a better understanding of both diseases and, importantly, the chance to find novel therapies, if drugs can be designed to specifically target one disease without increasing the risk of the other," Frain said.

The study was hailed by Dr. James Galvin, a professor of neurology, psychiatry, nursing and nutrition at NYU Langone School of Medicine in New York City.

"These findings are very important in light of recent studies of mouse models of [Alzheimer's] that showed possibly significant treatment effects on [Alzheimer's] pathology by a number of chemotherapeutic drugs, particularly those used to treat blood-related cancers [and] lung and liver cancers," Galvin said.

Dr. Catherine Roe, an instructor in neurology at Washington University School of Medicine in St. Louis, seconded the thought.

"If other scientists also find that chemotherapy is associated with a reduced risk of Alzheimer's disease, then it would be a good sign that we may be able to come up with a treatment that is effective in preventing Alzheimer's disease in the future," she said.

The data and conclusions of research presented at meetings should be considered preliminary until published in a peer-reviewed medical journal.